1. Field of the Disclosure
This disclosure relates generally to apparatus and methods for dispensing a viscous material on a substrate, such as a printed circuit board, and more particularly to a system and related methods for accurately dispensing material on the circuit board. This may be done within a dedicated dispenser system or a dispense unit mounted within a screen/stencil printer.
2. Discussion of Related Art
There are several types of prior art dispensing systems used for dispensing precise amounts of liquid or paste for a variety of applications. One such application is the assembly of integrated circuit chips and other electronic components onto circuit board substrates. In this application, automated dispensing systems are used for dispensing very small amounts, or dots, of viscous material onto a circuit board. The viscous material may include liquid epoxy or solder paste, or some other related material. Prior to performing a dispensing operation, the circuit board must be aligned or otherwise registered with a dispenser of the dispensing system. In one known method, this may be achieved by employing a vision system of the dispensing system to verify the location of landmarks, otherwise known as fiducials, or by locating known components on the circuit board. Specifically, to align the circuit board with the dispensing unit of the dispensing system, images of at least two fiducials or a known component are taken by a camera of the vision system. If the circuit board is out of position, a gantry capable of moving the dispenser may be manipulated to account for the actual position of the circuit board. In another embodiment, a support surface upon which the circuit board rests may be manipulated to accurately position the circuit board prior to performing a dispense operation.
One issue associated with dispensing is that a distance/relationship between the vision system and the dispensing unit may not be constant, due to potential non-linearity or non-orthogonality. This issue may lead to significant errors when attempting to accurately dispense material onto the circuit board. Every mechanical system has some amount of variance from a zero or perfect tolerance. A typical mechanical motion system is designed to achieve high tolerances of flat, straight and square over the travel of the system. These tolerances allow for some level of variation from system to system and from a perfect tolerance. A typical dispenser may employ a standard calibration routine, which will calibrate out these errors using the vision system of the dispenser. This calibration routine can be verified to correct these errors for the vision system positioning. However, the calibration routine does not sufficiently account for a relative distance between the vision system and the dispensing unit that is not constant over the entire travel of the system. In other cases, the dispense unit may be coupled to a separate mechanical motion system than the vision system, and thus a calibration of each system and the relationship between the two systems is required.